There is considerable need for surfaces with a black appearance, for example in the field of consumer articles such as watch cases, mobile phones, or bumpers on motor vehicles. In such applications, however, the outer, preferably jet-black or deep black appearance is not the only thing that is important; the resistance to external mechanical impacts is important, too. In particular, the surface should be scratch-resistant.
The German application DE3639469 describes a hard material layer with a decorative black appearance, which simultaneously has a high wear resistance. This hard material layer includes a first layer composed of an element from the groups IVa and Va of the periodic table of elements, a second layer that includes a nitride of this element of the first layer, a third layer that contains a carbide of the element, and a covering layer composed of a hard carbon layer, with carbide crystallites of the same element being embedded in this covering layer.
It is also known to use DLC layers. These have a black appearance and a high degree of hardness. However, conventional DLC layers have neutral gray values (L*=lightness) that lie in the range above 40. In this case and in the present description, the CIE 1976 L*a*b* color space, which is based on D65 standard lighting and a d/8° (=diffuse lighting, and measurement at under 8°), is taken as a basis. In connection with the present description, surfaces are considered to be jet-black surfaces if they have a neutral gray value L*=<40.
The black appearance of DLC layers is produced by its high absorption capacity. This is a result of the relatively high absorption coefficient of the material. This absorption coefficient, however, is accompanied by a high refractive index of between 2.1 and 2.3. As a result of this, when light strikes the surface of the DLC coating, a large jump in the refractive index, at the transition from air (n=1) into the layer causes a significant portion of the light to be reflected, thus resulting in a high L* value.
At this point, it would be possible to provide an optical anti-reflection layer on this surface. Such layers, however, are based on the principle of interference. Their reflection behavior is consequently on the one hand dependent on wavelength and on the other hand also dependent on the angle of incidence. In addition, the reflection behavior of such layers is highly dependent on the layer thickness produced. This can lead to significant problems, primarily when the geometries of components to be coated are not planar.
There is thus a need for a hard surface coating that produces a jet-black visual appearance that is preferably independent of the viewing angle.
The object of the present invention, therefore, is to disclose a hard surface coating that produces a jet-black visual appearance that preferably remains independent of the viewing angle.